Serial

There is space on PCB for a DB9 connector and a level converter (MAX3241 in a SO/SSOP/TSSOP package matches the pinout).
With sufficent soldering skills adding required components shouldn't be a problem.
A USB-to-serial (TTL, 3,3V) converter can also be used:
GND is at pin 5 of DB9 connector,
Txd is at pin 14 of U10,
Rxd is at pin 19 of U10.
Please note the little spot and triangle marking U10's first pin - the IC is upside down in relation to CPU.
Serial port is working at 115200, no parity.